Heretofore, as the still image photography of an x-ray in medical treatment, the main stream has been a film system in which an x-ray is irradiated on a patient, and its transmitted x-ray image is exposed on a film. The film has functions of displaying and recording information, and can be enlarged to a large area, and is high in gradation, and yet, it is light in weight and can be easily handled. Therefore, it is popularized throughout the world. On the other hand, left behind to be solved are complications requiring development processing, the problem of a location for storing over a long period of time, and the problem of manpower and time required for retrieval.
In the meantime, as a moving image radiography, the main stream has been an image intensifier (hereinafter, abbreviated as [I. I.]). Since the I.I. uses photo-multiplying effect inside the apparatus, in general, sensitivity is high, and it is excellent in view of a low dosage of exposure to radiation. On the other hand, shortcomings such as a distortion of the peripheral image due to optical influences, low contrast, and large size of the apparatus are pointed out. The I.I. has not only the transmitted image of the patient monitored by a doctor, but also can convert an analogue output of CCD into a digital signal so as to record, display or store the same. However, since a high gradation is required for diagnosis, even if the I.I. is used for the transmitted image, there are often the cases where the film is used in the still image photography.
In recent years, a demand for digitalization of the x-ray image has been increasing in the hospitals, and in place of the film, an imaging device disposed in a two-dimensional array pattern with a solid state image sensing device converting an electromagnetic wave such as a visible light and radiation into an electrical signal began to be used. This imaging device is called a FPD (Flat Panel Detector) for short.
Since this FPD can substitute an x-ray image with digital information, the image information can be transferred far away and instantaneously. Hence, an advantage is also offered in that, while being far away, an advanced diagnosis equal to a centrally located university hospital can be received. If the film is not used, an advantage is also offered in that a storage space for the film in the hospital can be eliminated. In the future, if an excellent image data processing technique can be introduced, a potential for an automatic diagnosis by using a computer without intermediary of a radiologist is greatly anticipated.
Further, a radiation imaging apparatus capable of radiographing a still image by using an amorphous silicon thin film semiconductor for the solid state image sensing device has been put to a practical use. As for this radiation imaging apparatus, a large area electronic display exceeding 40 cm square covering the size of a chest region of the human body is realized by using the manufacturing technique of the amorphous silicon thin film semiconductor. Since this manufacturing processing is relatively easy, in the future, realization of an inexpensive radiation imaging apparatus is anticipated. Moreover, since amorphous silicon can be made into a thin glass of not more than 1 mm, an advantage is offered in that a thickness as a detector can be made extremely thin.
Such radiation imaging apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. H08-116044. Further, in recent years, the radiographing of a moving image by such radiation imaging apparatus is being developed. If one set of such apparatus can be manufactured at a low cost, the still and moving images can be imaged, and therefore, it will be expected to become popular at the great many numbers of hospitals.
When the moving image is imaged by using such radiation imaging apparatus, a problem to be solved is that, as compared with the still image, a reading time is made shorter (frame rate is made faster) and S/N is improved. Hence, when the moving image is radiographed, a driving which is generally called as “pixel-addition” is sometimes performed. Usually, as against reading a single pixel as one pixel (hereinafter, this one pixel is referred to as “unit-pixel”), in the case of the pixel-addition, a plurality of pixels is put together and read as one pixel (hereinafter, this one pixel is referred to as “multi-pixel”). Hence, for example, when two pixels are bound, though the signal is doubled, the noise becomes only (√2) times, and therefore, as the S/N, as a S/N of 2/(√2)=(√2)≈1.4 times can be obtained.
Further, the pixel-addition includes a digital-addition and an analogue-addition. The digital-addition is read as usual and performs an A/D conversion, and after that, digitally binds up the unit-pixel and constitutes the multi-pixel. In contrast to this, the analogue-addition is a technique, in which analogue signals are bound up before A/D conversion, and after that, the A/D conversion is performed. The digital-addition is read as usual, and then, performs the A/D conversion, and therefore, though the reading time is not different from the case where the pixel-addition is not performed (hereinafter, referred to as “pixel non-addition”, the analogue-addition can shorten the reading time.
Further, as for a driving method for addition and reading the unit-pixel in a signal wiring direction, for example, it is disclosed in “Proceeding of SPIE, Vol. 5368, Item 721, 2004, Eric Beuville, Indigo System Corporation”. In this Non-Patent Document, the pixel-addition (averaging out of odd number lines and even number lines) in a signal wiring direction by a sampling and holding circuit unit at the preceding stage of an AD converter (ADC) is performed. The signal is averaged out, and the noise is increased by 1/(√2) times so that S/N=(√2) times. Thus, in the moving image radiographing, the driving by the pixel-addition can be said to be an important method for driving.
Further, though the radiation imaging apparatus performs various image processings for the radiographed image, the basic image processing among them includes an offset correction and gain correction, and a defect-correction. The offset correction is a processing for correcting a dark component of a photoelectrical conversion element and an offset component of a signal processing circuit unit. On the other hand, the gain correction is a processing for correcting the fluctuation in sensitivity of the photoelectric conversion element and the gain fluctuation in the signal processing circuit unit. This gain correction is performed such that, usually before radiographing an object, an x-ray is irradiated in a state in which no object exists, thereby performing radiographing, and by using the radiographed image as an image for gain correction, a division processing is performed for the image in which the object is radiographed, so that the correction is performed.
Further, the defect correction is a processing for correcting the pixel value of a defective pixel by using the pixel value of the defective pixel periphery. Although such radiation imaging apparatus comprises a semiconductor, when manufacturing the apparatus, due to a defect caused in the semiconductor and an influence of the dust adhered in the manufacturing process, there are often the cases where a defect is caused in the pixel. T0 manufacture the whole of a great many number of pixels comprising the radiation imaging apparatus without causing any defect is extremely difficult. Consequently, if the imaging apparatus including the defective pixel is not used, this will invite a reduction in yield ratio of the imaging apparatus. However, if the imaging apparatus including the defective pixel is used as it is, the quality of the image obtained by the radiographing is remarkably deteriorated due to the influence of the defective pixel.
Hence, in order to use the imaging apparatus including the defective pixel, heretofore, a correction technique for the defect pixel has been proposed. For example, the technique disclosed in Japanese Patent Publication No. H05-023551 corrects the defect by using an average rate of the pixel value in the periphery of the defective pixel.